1. Field of the Invention
The present invention relates to a method for integrally forming and manufacturing an one-piece forged hollow crank of a bicycle.
2. Description of the Related Art
A hollow crank of a bicycle made by a first conventional method includes multiple forged pieces which are assembled by a welding process. However, the hollow crank forged by multiple pieces is not integrally molded, thereby creating a weld effected zone in the material, so that the hollow crank has a worse stress tolerance. Therefore, the conventional hollow crank must use only alloys that are weldable and must design around the problems of the welding.
A second conventional method for making a hollow crank of a bicycle in accordance with the prior art shown in FIGS. 1 and 2 comprises the following steps:
(a) melting a material; PA1 (b) casting step; PA1 (c) opening mold; PA1 (d) removing sand core from a mold; PA1 (e) clearing residue material; PA1 (f) material surface treatment; and PA1 (g) forming a screw hole in the rear end of a crank.
However, such a conventional method has complex steps, thereby greatly consuming time for making the hollow crank. In addition, by such a method, it is easy to produce a defective product which cannot be controlled by the steps. A major disadvantage to this method is the relatively lower strength of casting alloys to forging alloys.
In the casting process, it is necessary to make a blank 10 before machining the two ends of the hollow crank. The blank 10 is cast by the mold 11 in conjunction with the sand core 12. After the mold is opened, the maker has to remove the cast duct 13 and the cast nozzle 14 by a cutter, and treating the originally rough surface (air vents, sand and mold lines) by a grinding machine so as to work the two ends of the blank 10. Therefore, such a method greatly increases the cost of fabrication, and consumes a great deal of time.